Fuel control system

ABSTRACT

A gas turbine fuel control system for controlling the acceleration of an engine during start-up which permits the engine to accelerate substantially along its required-to-run line. Fuel flow to the engine is controlled by a speed governor, and means are included to gradually increase the speed set point of the governor in relation to elapsed time of the start-up period.

This is a continuation application of Ser. No. 743,387 filed Nov. 19,1976 (abandoned).

BACKGROUND OF THE INVENTION

This invention relates to fuel control systems for delivering fuel flowto gas turbine engines, and relates more particularly to an improvedmethod and apparatus for controlling change in speed of the engine.

Performance of a gas turbine engine, including its accelerationcharacteristics, are limited by the onset of compressor instability orstall. Accordingly, fuel control systems for a gas turbine engine aredevised to, in one way or another, control fuel flow to avoid the regionof compressor instability. For fastest engine acceleration duringstarting, on the other hand, it has generally been proposed to controlfuel flow to the engine such that engine performance closely approachesbut does not encounter the region of instability or stall. In thismanner, it has been theorized that power is developed most rapidly bythe engine and maximum acceleration thus results. To accomplish thisresult, the majority of prior art fuel control systems are primarilyconcerned with sensing the proper engine performance parameters whichindicate the engine is operating at its near maximum performance beforeencountering stall, and accordingly scheduling fuel flow in response tothese sensed parameters. Thus, sensing temperature and pressure atvarious locations in the gas flow path through the engine, thenscheduling fuel flow in relation to these various sensed temperatures,pressures and other factors, has been a conventional approach toscheduling fuel flow to the engine during its acceleration. Byattempting to accelerate the engine closely to its region ofinstability, such prior art fuel control systems also were well suitedto avoid engine operation near its required-to-run line duringacceleration to minimize the possibility of "hung" starts. Conversely tothe instability or stall characteristics of a particular engine, therequired-to-run line of the engine dictates the minimum amount of fuelflow required to the engine in order to overcome the inertia thereof,load imposed thereon, and the like in order to maintain a given speed.Fuel flow less than that dictated by the required-to-run line causesdeceleration of the engine to a slower speed, and to accelerate theengine, obviously, the fuel flow to the engine must be above therequired-to-run line.

A serious drawback for fuel control systems operating to accelerate agas turbine engine relates to the inability to obtain sufficientlyaccurate measurements of the various parameters of pressure,temperature, etc. utilized in controlling fuel flow. Such sensingdevices have proved to be somewhat unreliable due to slow response timeand ultimate deterioration from the extreme environmental conditions towhich they may be exposed. Beyond this, it is also recognized that thesensing of such various engine parameters only gives a general"estimate" of how close the engine may be running to its instabilityregion for a given set of external conditions. Onset of compressorinstability or engine stall varies greatly dependent upon externalambient conditions such as temperature and pressure of the ambientairflow being received by the engine, as well as even the temperature ofthe fuel flow being delivered to the engine. Such changes inenvironmental conditions which markedly affect engine performancecharacteristics are highly pronounced in aircraft applications of gasturbine engines where the ambient conditions can vary drastically.Accordingly, fuel control systems have become more and moresophisticated to compensate for various changes in external conditionssuch that conditions of instability are avoided, yet while assuring theengine can still be maintained above its required-to-run line. Forinstance, the fuel control system must take into account sufficientvariations in engine performance characteristics so as to avoid "hotstarts" which are more likely to occur at high altitutde restarts of anaircraft mounted engine. "Hot starts" are a result of the engine runningnear its stall line by delivering a high volume fuel flow to the enginecombustion chambers such that a relatively high temperature at thedischarge of the engine turbine and downstream of the combustionchamber, as well as high temperature exhaust from the engine results.Excessive temperature on the turbine and exhaust components created bysuch "hot starts" can cause engine failure due to overheating of thesedownstream components. Exemplary disclosures of prior art systems may befound in U.S. Pat. No's. 3,011,310; 3,043,367; 3,085,619; 3,139,892;3,399,527.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide an improvedfuel control system and method wherein the engine acceleratessubstantially along or as closely adjacent its required-to-run lineregardless of ambient external conditions.

Another important object of the present invention is to provide a fuelcontrol system which accelerates a gas turbine engine without relianceupon sensing of various engine temperature and pressure parameters forits operation.

Another important object of the invention is to provide a novel fuelcontrol and method for a gas turbine engine which accelerates the engineas a function of the elapsed time of the engine start-up period.

Another important object of the invention is to provide fuel controlsand methods which control engine speed throughout acceleration to apreselected speed set point, along with method and means for changingthe pre-selected set point speed to produce a desired acceleration.

More particularly, the invention contemplates a fuel control systemhaving a speed governor responsive to engine speed to variably meterfuel flow delivered to the engine in a feedback loop arrangement inorder to maintain engine speed at the set point of the governor. Inconjunction with this, the invention contemplates method and means foradjusting the set point of the governor along a pre-selected schedule toproduce the desired change of speed of the engine at a controlled rate.Specifically, to accelerate the engine along its required-to-run line,the speed set point of the governor is changed gradually at a rate whichis somewhat slower than the corresponding acceleration of the enginewhen accelerating along its required-to-run line. In a preferredembodiment, such schedule change in the governor speed set point isaccomplished by a timing mechanism which is responsive to the elapsedtime of the engine start-up period.

These and other more particular objects and advantages of the presentinvention are specifically set forth or will become apparent from thefollowing detailed description of a preferred embodiment of theinvention, when read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially schematic, partially cross sectional view of a gasturbine engine and fuel delivery system constructed in accordance withthe principles of the present invention;

FIG. 2 is an exploded perspective view of the timing mechanismcontemplated by the present invention; and

FIG. 3 is a graphical representation of the fuel flow rate, W, deliveredto the engine versus engine speed, N.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now more particularly to the drawing, a gas turbine engine 10incorporates a combustion chamber 12 and a fuel delivery system 14 forcontrolling fuel flow to the combustion chamber. The fuel control system14 includes a housing 16 having a fuel pump 18 of the type including apair of intermeshing gears 19 which are driven by a shaft 20 operablycoupled to the rotating drive shaft of the engine by an appropriatedrive train schematically illustrated by dashed lines 22. Pressurizedfuel flow from pump 18 is delivered through its outlet port 24 to afirst fuel chamber 26 in housing 16. A second fuel chamber 28 in thehousing communicates with the combustion chamber 12 through anappropriate conduit 30. The present fuel control 14 can be used inconjunction with various other conventional fuel control devices asschematically illustrated by element 32. Chambers 26 and 28 communicatethrough a by-pass passage 34 having a minimum fuel control elementtherein, illustrated as a fixed orifice 36, which dictates the minimumrate of fuel flow delivered from chamber 26 to chamber 28 throughoutoperation of the engine. Housing 16 further includes an exhaust duct 38preferably interconnected with the inlet (not shown) of the pump 18. Ametering valve 40 movably mounted within housing 16 and urged leftwardlyby a biasing spring 42 acts to variably meter exhaust fuel flow fromchamber 26 through exhaust duct 38. The opposite sides of the pistonrepresented by valve 40 are respectively exposed to the pressure of fuelmaintained in chambers 26 and 28. In conventional manner therefore, thevalve 40 acts to maintain a substantially constant pressure differentialbetween chambers 26 and 28 by variably metering exhaust flow fromchamber 26 through duct 38.

The fuel control system further includes timing mechanism in the form ofa timing shaft 44 rigidly secured to rotate with fuel pump 18 and thusbe rotatably driven by the engine 10. Shaft 44 extends between first andsecond chambers 26 and 28 and includes a first internal flow passageway46 having opposite ends communicating with the chambers 26, 28 throughcross bores 47. The timing shaft 44 further includes a second passage ina form of a groove 48 on a portion of the external surface of shaft 44,which groove 48 continually communicates with second fuel chamber 28. Anon-rotating piston means 50 is disposed in chamber 26 to divide thelatter into first and second compartments 52, 54 on opposite sides ofpiston 50. Compartment 52 directly communicates with the pump outlet 24,while the second compartment 54 presents a substantially trapped fluidvolume. A light biasing spring 56 is mounted in compartment 54 to urgepiston 50 leftwardly against the force exerted by pump outlet fluidpressure in chamber 52, and a one-way check valve 58 permits one-wayfluid communication from second chamber 28 to second compartment 54while prohibiting reverse fluid flow therebetween. Firmly affixed topiston 50 is a non-rotating sleeve 60 disposed in substantiallysurrounding relationship to timing shaft 44. A cross duct 62 in sleeve60 permits periodic communication of the second compartment 54 with theexhaust path defined by groove 48 once each revolution of shaft 44.

Metering means in the form of a non-rotating element having a leftwardlyextending valve portion 64 is cooperable with rotating shaft 44 tovariably meter fluid flow from passage 46 through cross bore 47 tosecond chamber 28. Biasing means in the form of a single helical coilspring 66 urges valve 64 rightwardly as viewed in FIG.'S. 1 and 2 toincrease the rate of fuel flow into chamber 28. Spring 66 is grounded toan abutment shoulder 68 secured to sleeve 60 and thus operably carriedby piston 50. It will be apparent that the rightwardly directed biasingforce exerted by spring 66 varies in relationship to the axial movementof piston 50. A somewhat conventionally structured speed governorgenerally denoted by the numeral 70 operates to exert a leftward forceon valve 64 opposing that created by biasing spring 66. The governorincludes a disc 72 secured to rotate with timing shaft 44, and aplurality flyweights 74 are rotatably mounted at the periphery of disc72 such that the inwardly extending arms of the flyweights 74 areengagable with an inner rotating race 76 as shaft speed increases andthe flyweights 74 tend to rotate outwardly under centrifugal forces.Inner race 76 rotates with shaft 44, and through ball bearings 78transmits the axial force exerted by the flyweight arms 74 to anon-rotating outer race 80 which engages valve 64. Thus, upon increasein speed of shaft 44 the flyweight arms 74 tend to rotate radiallyoutwardly and exert a corresponding force tending to shift valve 64leftwardly to restrict fluid flow from passage 46 to chamber 28. Ifdesired, a valve 82 may be incorporated which is shiftable between itsclosed position illustrated blocking fluid communication betweencompartments 52 and 54, to a flow permitting position when wherein fluidpressure flow from first chamber 52 is directed into second chamber 54.

In operation, the performance characteristics of a conventional gasturbine engine can be most clearly understood by reference to FIG. 3which provides a plot of the fuel flow rate, "W", versus the enginespeed of operation, "N". Characteristically, the gas turbine engineoperates below an upper line "S" which indicates the conditions underwhich stall, surge or compressor instability can be encountered, andthus defines the upper limit of operation of the gas turbine engine. Theengine thus must operate under the line "S", and yet above the lowercurved line "R" which is indicative of the required-to-run linecharacteristic of the engine. The required-to-run line simply means theminimum amount of fuel flow required to maintain the engine at a certainengine speed. Each of the engine operating limit lines "S" and "R" canvary substantially dependent upon various changes in different operatingparameters, including the changes in external ambient conditions. Uponconsideration, it will be appreciated that the required-to-run line "R"is an indication of the overall inertia, load of the engine, etc., whichmust be overcome in order to maintain a certain engine speed. Therequired-to-run line specifies the minimum power required, at a givenset of ambient conditions and other parameters such as engine load,required to maintain a certain engine speed. It will also be apparentthat there exists a corresponding engine acceleration for the engine toaccelerate from a lower to a higher speed and yet while operatingsubstantially along or closely adjacent to this required-to-run line"R". Fuel control system 14 of the present invention is operable toassure that the engine does accelerate substantially along or closelyadjacent to the required-to-run line "R" regardless of changes in therequired-to-run line itself due to various external operating parameterssuch as external ambient conditions.

Fuel control system 14 operates in the following manner. Upon start-upof engine 10 through an external power source such as an electric motor,pump 18 gradually begins increasing in speed and displacing pressurizedfuel flow through its outlet 24 into compartment 52. Piston 50 islocated at its far leftward position upon start-up. In this position,fluid flow from compartment 52 into internal passage 46 of the timingshaft is substantially less than that allowed through the bypass orifice36. Accordingly, the size of bypass orifice 36 is the controllingparameter during the initial start-up phase of the engine. Asillustrated by line "A" in FIG. 3, the bypass orifice 36 isappropriately sized to allow engine start fuel flow into second chamber28 and thus into combustion chamber 12. During this initial phase ofstart-up and throughout engine operation, the pressure differentialbetween valve 40 operates to maintain a substantially constant pressuredifferential between compartment 52 and chamber 28. In this context itis noted that pump 18 is conventionally sized such that its outputdisplacement is somewhat greater than that required by the fuel deliverysystem such that a certain amount of return exhaust flow continuesthrough exhaust duct 38 to the inlet of the pump. In this manner thepump is so sized that the rate of fuel flow delivered therefrom is not afactor in the control function itself.

During the start-up period, the speed of the engine 10 is relatively lowand governor 70 does not exert sufficient force on valve 64 to controlengine operation. Accordingly, the rate of fuel flow as depicted by line"A" dictates that a fuel enriched initial start phase has occurred. Onceengine speed picks up to a certain minimum level, the speed governor 70comes on control by exerting a leftward force on valve 64 strong enoughto overcome the force of spring 66 and urge valve 64 to a positionmetering fuel flow from passage 46 into chamber 28. In combination withthe movement of the speed governor flyweights 74, the initialrevolutions of shaft 44 have allowed enough exhaust of fuel flow fromsecond compartment 54 to duct 62 and groove 48 into chamber 28 to allowslight rightward movement of piston 50 to clear communication betweenfirst compartment 52 and internal passage 46. Accordingly, at apre-selected engine speed, and at a pre-selected elapsed time of theengine start-up period as determined by pre-selected number ofrevolutions of timing shaft 44, fuel flow from compartment 52 passesprimarily through passage 46 to second chamber 28, and the position ofvalve 64 becomes the fuel controlling parameter.

By allowing only a single spurt of fuel from compartment 54 throughgroove 48 once each revolution of timing shaft 44, the rightwardmovement of piston 50 is quite slow, particularly in comparison to theresponse time of the feedback speed control presented by governor 72. Inthis context, the response time of governor 70 in shifting and movingthe valve 64 leftwardly and rightwardly is sufficiently faster than therelatively slow movement of piston 50, such that the valve 64 is capableof modulating the rate of fuel flow into chamber 28 so as to bringengine speed to a pre-selected level in accordance with the speedfeedback control system provided by governor 70. Thus, once the positionof valve 64 becomes the parameter controlling fuel flow, the engineoperating line A of FIG. 3 immediately is brought down to therequired-to-run line R by governor 70. Being a simple feedback loopcontrol system, it is apparent that governor 70 operates to reduce fuelflow to the engine so as to maintain engine speed at a certainpre-selected value, and thus dictates that the engine run on itsrequired-to-run line R regardless of parameters such as ambient pressureand temperature which affect the required-to-run line.

As the engine continues to operate, however, it will be apparent thatpiston 50 continues to shift at a slow speed gradually rightwardly as asmall spurt of fluid leaves compartment 54 during each revolution oftiming shaft 44. The rightward movement of piston 50 also moves abutment68 rightwardly increasing the compression of biasing spring 66. Thisincreases the speed set point and thus requires the engine to increaseits speed to produce a greater force on governor 70 to counteract theincreased biasing force of the more compressed spring 66. In this mannerit can be seen that as the piston 50 gradually moves rightwardly, theengine gradually accelerates along the required-to-run line. Once piston50 is shifted completely rightwardly, the governor 70 continues tomaintain a constant engine speed thereafter as the biasing spring 66exerts the precise force required to maintain engine speed at itsmaximum desired constant speed value.

It will be apparent that the timing piston 50 gradually increases thecompression of spring 66 and thus the speed set point of the governor 70in relation to the elapsed time of the start-up or acceleration period.

Thus, the position of abutment 68 which determines the compression ofspring 66, is in essence a signal generator producing a signal that isindicative of a desired engine speed. Accordingly, the value of thisdesired engine speed signal is automatically varied in relation to theelapsed time as sensed by movement of piston 50. Preferably, the speedof movement of piston 50 is sufficiently slow to assure that the engineruns along its required-to-run line in order to minimize unnecessaryheat buildup in the engine as exemplified by an increasing turbinedischarge or engine exhaust temperature. However, if desired, the speedof movement of piston 50 may be otherwise controlled as by the size ofduct 62 and/or groove 48 to allow the engine to accelerate at a fasterrate, i.e., substantially displaced from the required-to-run line R andcloser to the stall line S.

To assure that the engine accelerates substantially along or closelyadjacent the required-to-run line R, it is required that the speed ofmovement of piston 50 be less than the corresponding acceleration of theengine when accelerating along its required-to-run line. For instance,if it is assumed that an engine normally requires 40 seconds toaccelerate to 100% speed if operating completely along itsrequired-to-run line, then the duct 62 and groove 48 are sized such thatthe time required for piston 50 to move from its completely leftward toits completely rightward position be slightly more than 40 seconds;i.e., 45 seconds.

Once the engine is decelerated and/or stopped, second compartment 54 isautomatically refilled with fuel through one-way check valve 58 byvirtue of the leftward movement of piston 50 created by biasing spring56 once pressure in compartment 52 is relieved. Thus piston 50 isautomatically reset for another engine start. It will be apparent fromthe foregoing that the selected maximum engine speed may also be variedfrom that dictated by complete rightward movement of the piston 50,simply by modulating the inflow and outflow of fluid through compartment54 in order to hold piston 50 at a position other than its far rightwardposition during constant speed engine operation. For instance, a secondengine speed may be selected simply by actuating a valve 82 tointerconnect compartments 52 and 54. By relatively sizing these twocompartments and spring 56, the force of spring 56 can overcome theequal pressures maintained in compartments 52 and 54 to shift piston 50completely leftwardly so that fuel flow to the combustion chamber 12 iscontrolled solely by orifice 36. In response, the engine will bemaintained at a constant speed determined by the rate of fuel flowpermitted through orifice 36.

In the preferred embodiment fluid exhausts from compartment 54 tochamber 28 and on to the combustion chamber 12. This small fuel flow hasa negligible effect on the engine, but in any case can be taken intoaccount in controlling engine acceleration. If desired, flow fromcompartment 54 could be exhausted elsewhere, such as to exhaust duct 38.It will be also apparent that the present invention may be utilized inconjunction with other conventional engine fuel delivery controls asexemplified by controls 32, and that the fuel control system 14 may beeasily adapted to different engines simply by modifying the size, shape,or number of duct 62, groove 48, and volume of compartment 54.

It will be noted that the required-to-run line R approximates a secondorder curve while the varying force created by a single helical coilcompression spring 66 is normally approximating a first order function.To compensate for the second order curve R, a pair of helical coilcompression springs 66 may be incorporated. By incorporating such a pairof compression springs, the bypass orifice 36 may be made quite smallsuch that the governor 70 and valve 64 begin controlling engineoperation at a relatively low speed; i.e., 10% of rated maximum speed.On the other hand, it has been found preferable in many instances tosize orifice 36 and the movement of piston 50, such that the orifice 36controls engine operation up to approximately 45% of rated speed. Thisprovides a fuel enrichment during initial startup as discussedpreviously, and also then allows a single spring 66 to be utilized sincethe required-to-run line from the 40% speed point to 100% speed pointcan be closely approximated by a linear function such as that by spring66.

While it would appear that a somewhat slower engine acceleration wouldresult from the present invention, it has been found in actual practicethat since the engine operates substantially at its required-to-run linethroughout acceleration and at greater engine efficiency, theacceleration time is only slightly longer than if the engine wereoperating near curve S. Thus, the better engine efficiency at curve Rsubstantially offsets any time decrease afforded by operating near curveS. Tests have also shown a substantial reduction in turbine dischargetemperature during acceleration, and the automatic compensation forchanges in the parameters that affect the required-to-run line.

From the foregoing, it will be apparent that the present inventioncontemplates an improved apparatus and method for controlling fuel flowto a gas turbine engine by adjusting fuel flow to the engine during astart-up period in relation to the elapsed time of the start-up period.Further, it will be apparent that the invention contemplates a method ofaccelerating a gas turbine engine wherein engine speed is sensed, fuelflow to the engine is adjusted in relation to the sensed speed tomaintain the engine at a pre-selected speed, and then the pre-selectedspeed set point is increased at a selected rate. Preferably thisselected rate is slower than the corresponding acceleration of theengine when operating its required-to-run line. In other words, thepre-selected speed as dictated by the position of piston 50 and thecompression of spring 66, is increased at a rate slower than thecorresponding acceleration which occurs as the engine develops poweralong a minimum power requirement schedule defined as that powerrequired to overcome engine inertia and the like to maintain a certainengine speed. Further the invention contemplates such a method whereinfuel flow to the engine is adjusted to the difference between actual andselected engine speeds to minimize the difference therebetween by theaction of the governor 70 and speed feedback control, and the value ofthe desired engine speed signal is automatically changed at apre-selected rate, preferably the response rate of the speed feedbackcontrol being substantially faster than the pre-selected rate at whichthe value of the desired speed signal is changed. In the illustratedembodiment these methods and apparatus are carried out by increasing theforce created by compression spring 66 in order to cause engineacceleration. The spring force is increased by movement of the piston 50which is controlled in relation to the cumulative number of revolutionsof the timing mechanism represented by timing shaft 44.

The foregoing detailed description of a preferred embodiment of theinvention should be considered exemplary in nature and not as limitingto the scope and spirit of the invention as set forth in the appendedclaims.

Having described the invention with sufficient clarity that thoseskilled in the art may make and use it, I claim:
 1. In combination witha gas turbine engine fuel delivery system:metering means for adjustingthe rate of fuel flow delivered to said engine; governor meansresponsive to engine speed for exerting a first force on said meteringmeans in relation to engine speed; biasing means for exerting a second,opposing force on said metering means; time sensing means for sensingthe elapsed time of a preselected portion of engine operation, said timesensing means including mechanism rotatably driven by the engine, andmeans for sensing the cumulative number of revolutions of said mechanismduring said preselected portion of engine operation; and meansresponsive to said time sensing means for increasing said second forceas a predetermined function of said elapsed time to accelerate saidengine.
 2. In a gas turbine engine fuel control system having a passagedirecting fuel flow to the engine;metering means for adjusting the rateof fuel flow through said passage; governor means responsive to enginespeed and exerting a first force on said metering means in relation toengine speed; biasing means exerting a second force on said meteringmeans opposing said first force; mechanism including a timing shaftrotatably driven by the engine; and means operably associated with saidmechanism for increasing said second force in relation to the cumulativenumber of revolutions of said mechanism during start-up of the engine.3. A fuel control system as set forth in claim 2, further including afuel pump driven by said engine and directing pressurized fuel flow tosaid passage.
 4. A fuel control system as set forth in claim 3, whereinsaid means for increasing the second force includes piston means movableto alter the compression of said biasing means to adjust said secondforce, said piston operably associated with said shaft whereby pistonmovement is operably controlled by said shaft.
 5. A fuel control systemas set forth in claim 4, wherein said piston includes a shoulderengaging said biasing means, said biasing means including compressiblespring means extending between said metering means and said shoulder. 6.A fuel control system as set forth in claim 5, further including ahousing cooperable with said piston to define first and second fluidcompartments on opposite sides of said piston, said piston arrangedwhereby pressurized fuel from said pump is delivered to said firstcompartment to move said piston in a first axial direction contactingsaid second compartment and increasing said second force.
 7. A fuelcontrol system as set forth in claim 6, wherein said shaft defines afluid exhaust path periodically communicating with said secondcompartment to permit fluid escape therefrom and corresponding pistonmovement in said first direction in relation to the cumulative number ofrevolutions of said shaft.
 8. A fuel control system as set forth inclaim 7, further including a minimum flow bypass passage arranged inparallel relationship to said metering means whereby fuel from said pumpis delivered to the engine through said bypass passage in bypassingrelationship to said metering means.
 9. A fuel control system as setforth in claim 8, when said bypass passage is of a size such that fuelflow therethrough is capable of accelerating said engine toapproximately 45% of its rated speed.
 10. A fuel control system as setforth in claim 9, when said biasing means is a single helical coilcompression spring.
 11. A fuel control system as set forth in claim 8,further including an exhaust duct communicating with said fuel deliverypassage at a location upstream of said metering means, and valve meansfor controlling flow of fuel through exhaust duct to maintain asubstantially constant pressure differential across said metering means.12. A fuel control system as set forth in claim 7, when said governormeans includes a flyweight rotatably driven by said engine for urgingsaid metering means to move in a second axial direction in relation tospeed of rotation of said flyweight.
 13. A fuel control system as setforth in claim 12, when said flyweight is operably driven by said timingshaft.
 14. In combination with a gas turbine engine having a combustionchamber, a fuel delivery system comprising:a housing having first andsecond fuel chambers therein; means communication said second fuelchamber with said combustion chamber; a timing piston movably mounted inand traversing said first fuel chamber to divide the latter into firstand second compartments; a fuel pump having an outlet port communicatingwith said first compartment; a shaft operably driven by said engine andmounted in said housing, said shaft defining a first passagecommunicating said first compartment with said second fuel chamber, anda second passage defining a fluid exhaust; a sleeve carried by saidpiston in surrounding relationship to said shaft and having a ducttherein periodically communicating said second compartment with saidsecond passage once each revolution of said shaft to permitcorresponding movement of said piston; fuel metering means for variablyrestricting fuel flow from said first passage to said second fuelchamber; governor means responsive to engine speed for urging saidmetering means to move in a first direction; a reaction shoulder carriedby said piston; and compressible biasing means extending between saidreaction shoulder and said metering means for urging the latter to movein a second, opposite direction.
 15. A combination as set forth in claim14 further including valve means for selectively interconnecting saidfirst and second compartments.
 16. A combination as set forth in claim14, further including a one-way check valve extending between saidsecond chamber and said second compartment for permitting one-way fluidflow from said second chamber to said second compartment.
 17. Acombination as set forth in claim 16, further including a biasing springdisposed in said second compartment and engaging said piston to exert aforce thereon opposing the hydraulic force developed by fluid in saidfirst compartment.
 18. A combination as set forth in claim 14, whereinsaid second passage comprises a groove on the exterior of said shaftcontinuously communicating with said second chamber.
 19. In a gasturbine engine fuel delivery control system for controlling fuel flow tothe engine during a preselected portion of engine operation;sensingmeans for sensing the speed of said engine and for sensing the elapsedtime of said preselected portion of operation; and fuel control meansresponsive to said sensing means for controlling fuel flow to the engineto vary engine speed along a predetermined schedule as a function ofsaid elapsed time, said predetermined schedule being preselected wherebysaid engine accelerates substantially along its required-to-run-lineduring said preselected portion of operation.
 20. A system forscheduling fuel flow to a gas turbine engine during a start-up period,comprising:speed sensing means for sensing engine speed; signal meansfor producing a speed signal indicative of a desired engine speed; meansresponsive to said speed sensing means and said signal means foradjusting fuel flow to the engine to minimize the difference betweensaid sensed speed and said speed signal; time sensing means for sensingthe elapsed time of said start-up period; and means responsive to saidtime sensing means and operably associated with said signal means forchanging the value of said speed signal in relation to said elapsedtime, said engine having a characteristic required-to-run-line, saidmeans for changing the value of the speed signal operable to increasesaid value of the speed signal at a time rate sufficiently slow wherebythe engine accelerates substantially along said required-to-run-line.21. In a gas turbine engine having a minimum power requirement schedulefor overcoming engine inertia and the like to sustain different enginespeeds, said engine having a corresponding acceleration upon developingpower along said minimum power schedule, a fuel control systemcomprising:means for sensing actual engine speed; means for producing aspeed signal indicative of a desired engine speed; means responsive tosaid sensing means and said producing means for adjusting fuel flow tothe engine to minimize the difference between said actual and desiredengine speeds; and means for automatically increasing the value of saidspeed signal and thus said desired speed at a predetermined rate slowerthan said corresponding acceleration.
 22. In a fuel delivery system asset forth in claim 1, wherein said preselected portion of engineoperation is the start-up period of engine operation.